This invention relates to fiber optic sensors that may be incorporated on the end of a catheter to measure a blood parameter such as oxygen saturation, hematocrit or other parameter and, more particularly, to a disposable calibrator that offers multipoint calibration in a single package.
It has been well recognized that a fiber optic sensor requires calibration to compensate for manufacturing variations such as the light transmission of fibers, spacing between fibers at the tip of the sensor etc. There are also variations in the instrumentation such as the light output, the gain of the receiving circuit having the photodetectors, and the loss in the connection between the instrument and the fiber optic sensor. The calibration of the whole system requires a reference material that will provide a known amount of signal when the sensor is in close contact with the reference material. The algorithm within the signal processing in the instrument will generate a set of correction factors such that the instrument indicates the known value that is associated with the reference material.
Current known means of providing such calibration are as shown and described in U.S. Pat. No. 4,322,164 of Shaw et al and U.S. Pat. No. 4,744,656 of Moran et al. In each of the aforementioned patents, however, the calibration utilizes one substance to provide a known amount of signal and thus calibration is accomplished by selecting only one point on a calibration curve.
If the optical signal such as reflection light, scattered light, fluorescent light, is linearly proportional to the change of one physical parameter, such as the oxygen content of the blood, then one point calibration is sufficient for the required accuracy. However, if the signal is a function of more that one physical parameter, for example, the reflection light changes as the oxygen content and the hematocrit of the blood changes, then the system accuracy is not sufficient with one point calibration but requires two point or multi point calibration.
Accordingly, in such instances, the systems of the aforementioned patents are not sufficient to provide the accuracy for the measurement of blood parameters.